systemctl: hide first column with --plain instead of --no-legend

systemctl: hide first column with --plain instead of --no-legend

Hiding the first column, which may contain bullet circles, with --no-legend
is undocumented and potentially unexpected. On the other hand, not printing
bullet circles with --plain is documented so hiding the column with that
switch is sensible.

The combination "--full --no-legend --no-pager --plain" is appropriate for
automated processing of systemctl output.